Add VK_NULL_HANDLE vertex buffer binding support

[attackgoat]

This change enables VK_EXT_robustness2 features so that VK_NULL_HANDLE may be bound to a vertex buffer binding in order to render with just an index buffer. See #41

The first commit of this change adds a draft function for testing, bind_null_vertex_buffer. This function will very likely be replaced with something nicer before merge.

TODO:

• Test using fuzzer.rs
• Expand AnyBufferBinding or make a similar enum for these cases
• Gracefully handle unavailable features (test on such hardware)

[attackgoat]

Although this is entirely possible to do, I don't know of any applicable use-case for drawing from a null buffer. Closing until there is a reason to do this.

Drawing with no buffer specified (such as quads driven by gl_VertexIndex) is possible - see examples/multipass.rs.